inject_double_buffer.cc 8.64 KB
Newer Older
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36
/*!
 *  Copyright (c) 2017 by Contributors
 *
 * \brief Inject double buffering optimization for data fetch.
 * \file inject_double_buffer.cc
 */
#include <tvm/ir_pass.h>
#include <tvm/ir_visitor.h>
#include <tvm/ir_mutator.h>
#include "./ir_util.h"
#include "../arithmetic/compute_expr.h"

namespace tvm {
namespace ir {

// Detect double buffer variables.
class DoubleBufferDetector : public IRVisitor {
 public:
  void Visit_(const AttrStmt* op) final {
    if (op->attr_key == attr::double_buffer_scope) {
      touched_.insert(op->node.as<Variable>());
      IRVisitor::Visit_(op);
    } else {
      IRVisitor::Visit_(op);
    }
  }

  void Visit_(const Variable* op) final {
    if (touched_.count(op)) {
      touched_.erase(op);
    }
  }
  // The set of touched variable.
  std::unordered_set<const Variable*> touched_;
};

37 38 39 40 41 42 43 44 45 46 47 48

class StripDoubleBufferWrite : public IRMutator {
 public:
  Stmt Mutate_(const AttrStmt* op, const Stmt& s) final {
    if (op->attr_key == attr::double_buffer_write) {
      return Mutate(op->body);
    } else {
      return IRMutator::Mutate_(op, s);
    }
  }
};

49 50
class DoubleBufferInjector : public IRMutator {
 public:
51
  explicit DoubleBufferInjector(int split_loop)
52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83
      : split_loop_(split_loop) {}

  Stmt Inject(const Stmt& stmt) {
    DoubleBufferDetector detector;
    detector.Visit(stmt);
    if (detector.touched_.empty()) return stmt;
    for (const Variable* v : detector.touched_) {
      dbuffer_info_[v] = StorageEntry();
    }
    return ConvertSSA(this->Mutate(stmt));
  }

  Stmt Mutate_(const AttrStmt* op, const Stmt& s) final {
    if (op->attr_key == attr::storage_scope) {
      const Variable* buf = op->node.as<Variable>();
      auto it = dbuffer_info_.find(buf);
      if (it != dbuffer_info_.end()) {
        it->second.scope = op->value.as<StringImm>()->value;
        return Mutate(op->body);
      } else {
        return IRMutator::Mutate_(op, s);
      }
    } else if (op->attr_key == attr::double_buffer_scope) {
      return MakeProducer(op, s);
    } else {
      return IRMutator::Mutate_(op, s);
    }
  }

  Stmt Mutate_(const Allocate* op, const Stmt& s) final {
    auto it = dbuffer_info_.find(op->buffer_var.get());
    if (it != dbuffer_info_.end()) {
84
      it->second.stride = arith::ComputeReduce<Mul>(op->extents) * op->type.lanes();
85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111
      Stmt stmt = IRMutator::Mutate_(op, s);
      op = stmt.as<Allocate>();
      Array<Expr> new_extents{make_const(op->extents[0].type(), 2)};
      for (Expr e : op->extents) {
        new_extents.push_back(e);
      }
      CHECK(it->second.loop != nullptr);
      auto& alloc_nest = loop_allocs_[it->second.loop];
      alloc_nest.emplace_back(AttrStmt::make(
          op->buffer_var, attr::storage_scope,
          StringImm::make(it->second.scope),
          Evaluate::make(0)));
      alloc_nest.emplace_back(Allocate::make(
          op->buffer_var, op->type, new_extents, op->condition,
          Evaluate::make(0)));
      return op->body;
    } else {
      return IRMutator::Mutate_(op, s);
    }
  }

  Stmt Mutate_(const For* op, const Stmt& s) final {
    loop_nest_.push_back(op);
    Stmt stmt = IRMutator::Mutate_(op, s);
    auto it = loop_pre_.find(op);
    if (it != loop_pre_.end()) {
      const For* old_loop = stmt.as<For>();
112 113 114 115 116 117
      if (split_loop_ != 0) {
        // Explicitly unroll the loop
        CHECK(split_loop_ % 2 == 0 || split_loop_ == 1)
            << "It is better to split with multiple of 2";
        CHECK(is_zero(old_loop->min));
        Expr zero = old_loop->min;
118 119
        Expr new_ext = arith::ComputeExpr<Sub>(
            old_loop->extent, make_const(old_loop->loop_var.type(), 1));
120 121 122 123
        Expr factor = make_const(new_ext.type(), split_loop_);
        Expr outer_ext = arith::ComputeExpr<Div>(new_ext, factor);
        Expr tail_base = arith::ComputeExpr<Mul>(outer_ext, factor);
        Var outer_var(old_loop->loop_var->name_hint + ".outer", old_loop->loop_var.type());
124
        std::unordered_map<const Variable*, Expr> vmap;
125
        std::vector<Stmt> loop_seq;
126
        for (int32_t i = 0; i < split_loop_; ++i) {
127 128 129 130 131 132 133 134 135
          vmap[old_loop->loop_var.get()] = outer_var * factor + make_const(factor.type(), i);
          loop_seq.emplace_back(Substitute(old_loop->body, vmap));
        }
        Stmt loop = For::make(
            outer_var, zero, outer_ext, old_loop->for_type, old_loop->device_api,
            MergeSeq(loop_seq));
        // tail
        std::vector<Stmt> tail_seq;
        Stmt tail_body = StripDoubleBufferWrite().Mutate(old_loop->body);
136
        for (int32_t i = 0; i < split_loop_; ++i) {
137 138 139 140 141 142 143
          Expr idx = tail_base + make_const(tail_base.type(), i);
          vmap[old_loop->loop_var.get()] = idx;
          tail_seq.emplace_back(
              IfThenElse::make(idx < old_loop->extent,
                               Substitute(tail_body, vmap)));
        }
        stmt = Block::make(loop, MergeSeq(tail_seq));
144 145 146 147 148 149 150 151 152 153 154 155 156 157 158 159 160 161
      }
      stmt = Block::make(MergeSeq(it->second), stmt);
    }
    it = loop_allocs_.find(op);
    if (it != loop_allocs_.end()) {
      stmt = MergeNest(it->second, stmt);
    }
    loop_nest_.pop_back();
    return stmt;
  }

  Stmt Mutate_(const Store* op, const Stmt& s) final {
    Stmt stmt = IRMutator::Mutate_(op, s);
    op = stmt.as<Store>();
    auto it = dbuffer_info_.find(op->buffer_var.get());
    if (it != dbuffer_info_.end()) {
      const StorageEntry& e = it->second;
      CHECK(in_double_buffer_scope_);
162
      CHECK(e.stride.defined());
163 164
      return Store::make(op->buffer_var,
                         op->value,
165
                         e.switch_write_var * e.stride + op->index,
166 167 168 169 170 171 172 173 174 175 176 177
                         op->predicate);
    } else {
      return stmt;
    }
  }

  Expr Mutate_(const Load* op, const Expr& e) final {
    Expr expr = IRMutator::Mutate_(op, e);
    op = expr.as<Load>();
    auto it = dbuffer_info_.find(op->buffer_var.get());
    if (it != dbuffer_info_.end()) {
      const StorageEntry& e = it->second;
178
      CHECK(e.stride.defined());
179 180 181
      CHECK(e.switch_read_var.defined());
      return Load::make(op->type,
                        op->buffer_var,
182
                        e.switch_read_var * e.stride + op->index,
183 184 185 186 187 188 189 190 191 192 193 194 195 196 197 198 199 200 201 202 203 204 205 206 207 208 209 210 211 212 213 214 215 216 217 218 219 220 221 222 223 224 225 226 227 228 229
                        op->predicate);
    } else {
      return expr;
    }
  }

  Expr Mutate_(const Variable* op, const Expr& e) final {
    CHECK(!dbuffer_info_.count(op));
    return e;
  }

 private:
  Stmt MakeProducer(const AttrStmt* op, const Stmt& s) {
    const VarExpr buffer(op->node.node_);
    CHECK_NE(loop_nest_.size(), 0U)
        << "Double buffer scope must be inside a loop";
    auto it = dbuffer_info_.find(buffer.get());
    if (it == dbuffer_info_.end()) {
      LOG(WARNING) << "Skip double buffer scope " << op->node;
      return Mutate(op->body);
    }
    StorageEntry& e = it->second;
    e.loop = loop_nest_.back();
    Expr zero = make_const(e.loop->loop_var.type(), 0);
    Expr one = make_const(e.loop->loop_var.type(), 1);
    Expr two = make_const(e.loop->loop_var.type(), 2);
    Expr loop_shift = e.loop->loop_var + one;
    e.switch_write_var = Var(e.loop->loop_var->name_hint + ".db",
                             e.loop->loop_var.type());
    e.switch_read_var = e.loop->loop_var % two;
    in_double_buffer_scope_ = true;
    Stmt body = Mutate(op->body);
    in_double_buffer_scope_ = false;
    std::unordered_map<const Variable*, Expr> vmap;
    vmap[e.switch_write_var.get()] = zero;
    vmap[e.loop->loop_var.get()] = zero;
    loop_pre_[e.loop].emplace_back(Substitute(body, vmap));
    vmap[e.loop->loop_var.get()] = loop_shift;
    vmap[e.switch_write_var.get()] = loop_shift % two;
    body = Substitute(body, vmap);
    body = AttrStmt::make(buffer, attr::double_buffer_write, 1, body);
    body = IfThenElse::make(loop_shift < e.loop->extent, body);
    return body;
  }
  // Storage entry for those who need double buffering.
  struct StorageEntry {
    // The size of the buffer
230
    Expr stride;
231 232 233 234 235 236 237 238 239 240
    // The loop we need
    const For* loop{nullptr};
    // The switch variable.
    VarExpr switch_write_var;
    // The switch variable for reading.
    Expr switch_read_var;
    // The storage scope.
    std::string scope;
  };
  // Whether split loop
241
  int32_t split_loop_;
242 243 244 245 246 247 248 249 250 251 252 253 254
  // Whether we are inside double buffer scope.
  bool in_double_buffer_scope_{false};
  // The current loop next
  std::vector<const For*> loop_nest_;
  // The allocs to be appended before the loop
  std::unordered_map<const For*, std::vector<Stmt> > loop_allocs_;
  // The stmt to be appended before the loop
  std::unordered_map<const For*, std::vector<Stmt> > loop_pre_;
  // The allocation size of the buffer
  std::unordered_map<const Variable*, StorageEntry> dbuffer_info_;
};


255
Stmt InjectDoubleBuffer(Stmt stmt, int split_loop) {
256 257 258 259
  return DoubleBufferInjector(split_loop).Inject(stmt);
}
}  // namespace ir
}  // namespace tvm